1. Field of the Invention
This invention relates to a semiconductor device and a method of manufacturing the semiconductor device, and more particularly to a semiconductor device capable of alleviating damage to the inside of the substrate caused by optical heating and a method of manufacturing the semiconductor device.
2. Description of the Related Art
The performance of LSI has been increased by increasing the degree of integration, or by miniaturizing the elements constructing LSI. As the element size is made smaller, the parasitic resistance and the short channel effect become greater. Therefore, the formation of a low resistance, shallow pn junction is becoming more important.
A shallow impurity diffusion region can be formed by implanting ions at low acceleration energy and optimizing subsequent annealing. To decrease the resistance of the impurity diffusion region, it is necessary to perform an anneal at high temperature to activate impurity ions.
Boron (B) ions, phosphorus (P) ions, or arsenic (As) ions are used as impurity ions for ion implantation. These impurity ions have a large diffusion coefficient in silicon (Si). Therefore, in RTA (Rapid Thermal Anneal) using a halogen lamp, an inward diffusion and outward diffusion of impurity ions take place, which makes the formation of shallow impurity diffusion regions difficult gradually.
The inward diffusion and outward diffusion can be suppressed by lowering the anneal temperature. However, if the anneal temperature is lowered, the activation rate of the impurity ions decreases significantly. Therefore, even if the approach of lowering the anneal temperature, it is difficult to form a shallow impurity diffusion region.
As described above, it is difficult to form a low resistance, shallow (20 nm or less) impurity diffusion region by the RTA process using a conventional halogen lamp.
To overcome this problem, a flash lamp annealing method using a flash lamp into which noble gas, such as xenon (Xe), is sealed has recently been examined as means for supplying instantaneously the energy necessary to activate impurity ions.
The ½ pulse width of a flash lamp is about 10 milliseconds. Therefore, when the flash lamp annealing method is used, the time during which the wafer surface is kept at high temperature is very short, with the result that the impurity ions implanted into the wafer surface hardly diffuse. Accordingly, it is possible to activate the impurity ions with almost no change in the distribution of the impurity ions implanted in the wafer surface.
However, the conventional flash lamp annealing method has the following problem.
To achieve a sufficient annealing effect, power intensity as high as 20 J/cm2 or more is required. In addition, the temperature at the wafer surface rises abruptly. As a result, a temperature difference develops between the right side and reverse side of the wafer, leading to an increase in the thermal stress inside the wafer. Such an increase in the thermal stress causes damage (heat damage), such as slips or cracks (breaks), in the wafer. Such heat damage to the wafer leads to a decrease in the production yield.
The heat damage to the wafer can be avoided by lowering the temperature at which the wafer is preheated or the irradiation energy density of the flash lamp before turning on the flash lamp. In that case, however, a sufficient activation of impurity ions cannot be expected.